High strength thin steel sheet excelling in weldability and process for producing the same

ABSTRACT

Provided are a high strength thin steel sheet having tensile strength of about 800 MPa or more, and a manufacturing method thereof. The thin steel sheet is mainly used for construction materials, home appliances, and automobiles. The thin steel sheet has excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio. The thin steel sheet includes, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, SoLAl: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and includes Fe and other inevitable impurities as a remainder. Here, Si, Mn, B, Sb, P, and S meet conditions of 5&lt;(Si/Mn+150B)/Sb&lt;20 and C+Mn/20+Si/30+2P+4S&lt;0.27. Also, the manufacturing method can secure workability of the thin steel sheet.

TECHNICAL FIELD

The present invention relates to a high strength thin steel sheet havinga tensile strength of about 800 MPa or more, mainly used forconstruction materials, home appliances, and automobiles, and amanufacturing method thereof, and more particularly, to a high strengththin steel sheet having excellent plating characteristic, weldingcharacteristic, bending workability, and hole expansion ratio (HER) aswell as high tensile strength, and a manufacturing method thereof.

BACKGROUND ART

Recently, a steel sheet for automobiles has required even higherstrength to improve fuel economy or durability. A high strength steelsheet having a high strength of about 800 MPa is increasingly used for acar's body structures or a reinforcing material in aspects of collisionsafety and passenger protection. However, since the high strength of asteel sheet causes a reduction in moldability and a weldingcharacteristic, the development of a material for complementing thisproblem is highly required. In response to this requirement, steelsheets of various composite structures such as ferrite-martensite dualphase steel or transformation-induced plasticity (TRIP) steel sheetusing transformation-induced plasticity of retained austenite have beendeveloped up to now.

For example, Japanese Laid-Open Patent Publication No. 6-145892 proposesa method for manufacturing a steel sheet having excellent moldability bycontrolling chemical components and an amount of retained austenite.Japanese Patent No. 2660644 and Japanese Patent No. 2704350 propose amethod for manufacturing a high strength steel sheet having pressmoldability by controlling chemical components and fine structures ofthe steel sheet. Also, Japanese Patent No. 3317303 proposes a steelsheet including retained austenite of 5% or more and having an excellentmoldability, particularly, excellent local elongation. However, most ofthe above-described related arts have been developed to improveductility. Sufficient considerations of bending workability, a holeexpansion ratio, welding characteristic, etc., which are importantstandards during actual part processing, have not been made.

Among the required characteristics of a steel sheet, a most crucialcharacteristic of a steel sheet used for a car's body structure or areinforcing material mainly requiring a steel sheet of high strength of800 MPa or more is a spot welding characteristic. The steel used for acar's body structure or a reinforcing material protects passengers byabsorbing collision energy during a collision. If the strength of a spotwelded portion is not sufficient, the portion will be destroyed and cut,so that a sufficient level of collision energy absorption cannot beobtained. For technology regarding high strength steel sheet withconsideration of a welding characteristic, there exists JapaneseLaid-Open Patent Publication No. 2003-193194, but it does not meet awelding characteristic actually required by the market.

Also, Japanese Laid-Open Patent Publication No. 2005-105367 proposestechnology of securing a welding characteristic and ductility for steelof 780 MPa or more. In the case of manufacturing a steel sheet having ahigh strength of 800 MPa or more in a real process, a cold rollingcharacteristic is remarkably reduced due to the high strength of a hotstrip, which is an intermediate material. Also, since a rapid coolingheat treatment condition should be applied during an annealing process,workability is also remarkably reduced. Japanese Laid-Open PatentPublication No. 2005-105367 has no sufficient consideration of theseproblems.

DISCLOSURE OF INVENTION Technical Problem

The present invention has been made to solve the foregoing problems withthe prior art, and therefore an object of the present invention is toprovide a steel sheet having excellent plating characteristic, weldingcharacteristic, bending workability, and hole expansion ratio inmanufacturing a thin steel sheet having high tensile strength of 800 MPaor more. Also, another object of the present invention is to provide amethod of securing workability of a steel sheet.

Technical Solution

According to an aspect of the present invention, there is provided asteel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn:1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N:0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, andincluding at least one material selected from the group consisting ofTi: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Feand other inevitable impurities as a remainder, wherein Si, Mn, B, Sb,P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 andC+Mn/20+Si/30+2P+4S<0.27.

According to another aspect of the present invention, there is provideda method for manufacturing a steel sheet, the method including:reheating a slab of the steel sheet including, in weight %, C:0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% orless, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B:0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one materialselected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%,and Mo: 0.003-0.08%, and including Fe and other inevitable impurities asa remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27, and rolling andwinding the slab at a temperature of a finish rolling exit side betweenthe Ar₃ transformation point and 950° C.; pickling a wound hot rolledsteel sheet and performing cold rolling on the same at a reduction ratioof 40-80%; and performing continuous annealing on an obtained coldrolled steel sheet at a temperature range of 740-860° C., cooling thecold rolled steel sheet down to 250-600° C. at a cooling rate satisfyinga condition of −5 Log CR+25C−17Si+40Cr+13,000B>30 in a cooling raterange of 3-150° C./s, and cooling the same at a cooling rate of 5°C./minute or more.

The steel sheet may have a structure including at least one selectedfrom the group consisting of bainite and bainitic ferrite occupying 40%or more, and ferrite and martensite phases occupying the remainder.

ADVANTAGEOUS EFFECTS

The present invention can provide a steel sheet having excellent platingcharacteristic, welding characteristic, bending workability, and holeexpansion ratio while having high a tensile strength of about 800 MPa ormore, and a manufacturing method thereof that can secure themanufacturability of the steel sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

A steel sheet includes, in weight %, C: 0.02-0.20%, Si: 1.5% or less,Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N:0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, andincluding at least one material selected from the group consisting ofTi: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Feand other inevitable impurities as the remainder, and Si, Mn, B, Sb, P,and S meet conditions of 5<(Si/Mn+150B)/Sb<20 andC+Mn/20+Si/30+2P+4S<0.27.

Also, a method for manufacturing the steel sheet includes: reheating aslab of the steel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5%or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al:0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb:0.001-0.10%, and including at least one material selected from the groupconsisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, andincluding Fe and other inevitable impurities as the remainder, whereinSi, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 andC+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at atemperature of a finish rolling exit side between Ar₃ transformationpoint and 950° C.; pickling a wound hot rolled steel sheet andperforming cold rolling on the same at a reduction ratio of 40-80%; andperforming continuous annealing on an obtained cold rolled steel sheetat a temperature range of 740-860° C., cooling the cold rolled steelsheet down to 250-600° C. at a cooling rate satisfying a condition of −5Log CR+25C−17Si+40Cr+13,000B>30 in a cooling rate range of 3-150° C./s,and cooling the same at a cooling rate of 5° C./minute or more.

The steel sheet includes at least one selected from the group consistingof bainite and bainitic ferrite occupying 40% or more, and ferrite andmartensite occupying the remainder.

Hereinafter, the present invention will be described in detail.

Carbon (C) is preferably in a weight % of 0.02-0.20 weight %(hereinafter simply referred to as %).

Carbon in steel is an element added in order to strengthen atransformation structure. However, when an amount of C exceeds 0.20%, ahole extension characteristic and a welding characteristic is reduced.On the other hand, when an amount of C is less than 0.02%, it isdifficult to secure strength.

Silicon (Si) is preferably in 1.5% or less.

Silicon in steel is an element that can be effectively used in order toimprove strength. However, since silicon not only causes surface scaledefects but also reduces the surface characteristic of a plated steelsheet in connection with a surface characteristic. Also, silicon reducesa chemical treatment characteristic. Therefore, generally, siliconcontent is limited to 1.0% or less. Since recent progress in platingtechnology allows silicon content in steel up to 1.5% without a greatproblem during a manufacturing process, the content is limited to 1.5%or less.

Mn is preferably in 1.5-3.0%.

Mn in steel is an element having a very high solid-solutionstrengthening effect and simultaneously, facilitates the formation of acomposite structure including ferrite and martensite. When Mn content insteel is less than 1.5%, it is difficult to secure the strength requiredby the present invention. When Mn content exceeds 3.0%, there is a highpossibility that problems in a welding characteristic and a hot rollingcharacteristic will occur.

P is preferably in 0.001-0.10%.

P in steel is an element having an effect of strengthening the steel.When P content is less than 0.001%, not only can the strengtheningeffect can be secured but a problem in manufacturing costs may also begenerated. On the other hand, when P content is excessively added, pressmoldability may reduce and brittleness of steel may occur.

S is preferably in 0.010% or less.

S in steel is an impurity element, hindering the ductility and weldingcharacteristic of a steel sheet. When S content in steel exceeds 0.01%,there is a high possibility of hindering the ductility and weldingcharacteristic of a steel sheet.

Sol.Al is preferably in 0.01-0.4%.

Sol.Al in steel is an effective element to combine with oxygen in steelto perform a deoxidation operation, distribute carbon inside ferrite toaustenite to improve martensite hardening ability. When Sol.Al contentis less than 0.01%, such an effect cannot be secured. On the other hand,when Sol.Al content exceeds 0.4%, such an effect is saturated andmanufacturing costs may increase.

N is preferably 0.020% or less.

N in steel is an element that effectively stabilizes austenite. When Ncontent in steel exceeds 0.020%, the stability of austenite greatlyincreases to prevent the formation of bainite, which is a fine structureintended by the present invention.

Cr is preferably in 0.3-1.5%.

Cr in steel is an element added to improve the hardening ability ofsteel and to secure high strength. In the present invention, Cr plays animportant role of facilitating the formation of bainite. When Cr contentin steel is less than 0.3%, such an effect is difficult to secure. WhenCr content in steel exceeds 1.50%, such an effect is saturated and isdisadvantageous economically.

Boron (B) is preferably in 0.0010-0.0060%.

Boron in steel is an element used to delay the transformation ofaustenite into pearlite during a cooling process of an annealingprocess. B is added as an element of suppressing forming of ferrite andfacilitating forming of bainite. However, when B content in steel isless than 0.0010%, such an effect is difficult to obtain. When B contentin steel exceeds 0.0060%, excessive B is inspissated on a surface tocause deterioration in plating adhesion.

Sb is preferably 0.001-0.1%.

Sb in steel is an indispensable element added in order to secure anexcellent plating characteristic in the present invention. Sb has anoutstanding effect in suppressing surface inspissation of oxides such asMnO, SiO₂, Al₂O₃, etc. to reduce surface defects, and suppressingcoarsening of surface inspissation materials by temperature rise and achange in a hot rolling process. When Sb content is less than 0.001%,such an effect is difficult to secure, and even when an added amountcontinuously increases, such an effect does not increase greatly andproblems of manufacturing costs and moldability reduction may begenerated. Therefore, Sb content is limited to 0.001-0.1%.

According to the present invention, one or two or more materialsselected from Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08% areadded to the steel formed of the above elements to achieve a strengthincrease and miniaturization of grain diameters.

When an added amount of Ti, Nb, and Mo is less than 0.003% in its lowerlimit, an effect of achieving a strength increase and miniaturization ofgrain diameters is difficult to secure. When an added amount exceeds0.08% in its upper limit, manufacturing costs may be increased andductility may be remarkably reduced due to excessive eduction materials.

The steel of the present invention is formed with Fe and otherinevitable impurities as the remainder besides the above-describedelements.

According to the present invention, an alloy constituent ratio of Si,Mn, B, Sb, P, and S may satisfy the following Math Figures 1 and 2 indesigning an alloy of a steel sheet having the above-described componentranges.

MathFigure1

5<(Si/Mn+150B)/Sb<20  [Math.1]

MathFigure2

C+Mn/20+Si/30+2P+4S<0.27  [Math.2]

Math Figure 1 is a component relation that can secure surface quality,obtained as an empirical numerical value. That is, Mn, Si, and B insteel are elements having a characteristic of forming inspissationmaterials on a surface during an annealing process. As inspissationmaterials of these elements increase, a plating characteristic isreduced. On the other hand, since Sb hinders a grain boundary diffusionof the above surface inspissation elements, Sb is very advantageous inan aspect of surface quality. For example, when a value calculated byMath Figure 1 is between 5 and 20, it means that a good surface qualitycan be secured.

Meanwhile, Math Figure 2 is a component relation that can secure adesirable welding characteristic, obtained as an empirical numericalvalue. That is, C, Mn, Si, P, and S in steel raise a carbon equivalent.As well known in the art, when a carbon equivalent is high, a weldingcharacteristic is reduced. Setting a condition by which a welding defectis not generated during spot welding, which is a welding methodprimarily performed when steel of the present invention is used throughrepeated experiments provides Math Figure 2. When a value calculatedusing Math Figure 2 exceeds 0.27, it means that there is a highpossibility that a welding defect may be generated.

A steel sheet of the present invention has a structure in which one ormore selected from bainite and bainitic ferrite occupy 40% or more, andferrite and martensite phases occupy the remainder. Ferrite andmartensite may occupy 25% or less and 35% or less, respectively.

Hereinafter, a method for manufacturing steel sheet formed of the abovecomponents using a cold rolled steel sheet will be described in detail.

A slab whose components have been formed using the above-described alloydesigning method is reheated and hot rolling is performed. Finishrolling in the hot rolling may be performed at a temperature of an exitside between the Ar₃ transformation point and 950° C. That is, at a hotfinish rolling temperature below the Ar₃ transformation point, there isa high possibility that hot transformation resistance rapidly increases,and a problem in manufacturing may be generated. At a temperatureexceeding 950° C., not only may excessively thick oxidation scalesoccur, but there is also high possibility that a steel sheet may becoarsened.

A hot rolled steel sheet manufactured using the above process is pickledand cold-rolled.

A reduction ratio of the cold rolling may be 40-80%. When a reductionratio is less than 40%, recrystallization driving force is weakened, sothat there is possibility that a problem may be generated in obtaininggood recrystalline grain. When a reduction ratio exceeds 80%, a rollingload increases rapidly.

The above obtained cold rolled steel sheet is continuously annealed at atemperature of preferably 740-860° C. When temperature is less than 740°C. during continuous annealing, a danger that non-recrystallizationgrain is formed increases. When temperature exceeds 860° C., a largegrain may be formed and simultaneously, a strip passing ability may bedefective due to a high temperature annealing operation.

After the continuous annealing, the cold rolled steel sheet iscontinuously cooled down to a temperature of 250-600° C. at a coolingrate allowing a value calculated by following Math Figure 3 to exceed 30within the cooling rate (CR) of 3-150° C./s, and then is graduallycooled down at a cooling rate of 5° C./min. or more. A high strengththin steel sheet having tensile strength of 800 MPa and having goodplating characteristic, welding characteristic, and hole expansion ratiocan be easily manufactured by continuously annealing the thin steelsheet under the above condition.

MathFigure3

5 Log CR+25C−17S+40Cr+13,000B>30  [Math.3]

where CR is a cooling rate.

When a cooling rate is lowered to less than 3° C./s after the continuousannealing, ferrite or pearlite is formed, so that strength intended bythe present invention is difficult to secure. Also, if the cooling rateis too higher than 150° C./s, hard phase of martensite, etc. isexcessively formed, so that bending workability and a hole expansionratio is greatly reduced, and also reduction in a strip passing abilitydue to a shape defect during a process is greatly worried. Therefore,cooling may be performed in the cooling rate (CR) of 3-150° C./s asdescribed above.

Also, to accomplish excellent bending workability and hole expansionratio, which are the characteristics of steel according to the presentinvention, a cooling rate allowing a value calculated by Math Figure 3to exceed 30 should be applied. That is, when a value calculated by MathFigure 3 is less than 30, bainite or bainitic ferrite phase, in whichthe steel of the present invention intends to obtain as its finestructure, is difficult to obtain by as much as 40% or more. When thebainite-based structure is obtained by as much as 40% or more, a producthaving excellent bending workability and hole expansion ratio whilehaving high strength of about 800 MPa, which are the characteristics ofthe steel according to the present invention, can be manufactured.

Meanwhile, a cooling final temperature for a cooling operation may be atemperature between 250 and 600° C. When a cooling final temperature isless than 250° C., a danger that a large amount of martensite will beformed increases. When a cooling final temperature exceed 600° C., alarge amount of soft phases of ferrite or pearlite, etc., are formed, sothat an intended material is difficult to accomplish.

The above-described manufacturing method can be likewise applied to aplated product such as a hot-dip galvanized material (GI) and agalvannealed material (GA) as well as a cold rolled steel sheet.

MODE FOR THE INVENTION

Hereinafter, the present invention is described in more detail using anembodiment thereof.

As illustrated in Table 1, a slab having the component composition ofthe present invention is heated to a temperature of 1200° C. andextracted, and then rolling is performed at a cold reduction ratio of55% using, as a material, a hot rolled steel sheet manufactured byhot-rolling the slab under a condition of a finish rolling temperatureof 900° C. Continuous annealing heat treatment is performed (CR) at theannealing temperature and cooling condition of Table 2. A plated productis manufactured by performing hot-dip galvanizing (GI) and galvannealing(GA) processes. Conditions and galvannealing process time applied duringcontinuous annealing are given below.

-   -   Annealing furnace atmosphere: N₂-10% H₂O (dew point −32° C.)    -   Annealing furnace heating rate: 3° C./sec    -   Annealing time: 90 sec    -   Plating temperature: 460° C.    -   Galvannealing time: 24 sec (in case of GA product)

As illustrated in Table 2, plating characteristics (appearance andadhesion characteristic) and the quality of a material (tensilestrength, hole expansion ratio, and bending workability) are measuredand results thereof are shown together with a comparison material.

In Table 2, a plated appearance is represented by non-plating or ◯ for acase not including other plating defects. A defect name is written for acase where a plating defect is generated.

In Table 2, a plating adhesion appraisal has been made in the followingway, in which: a plated sheet is cut off by 20 mm×50 mm, a bending testis performed on the plated sheet, the plated sheet is unfolded again, atape is attached on the folded portion of the plated sheet, and thewidth of a plated layer detached from the plated sheet is appraisedusing the following criteria.

⊚: No detached plating or width of detached plating is within about 1 mm

◯: Width of detached plating is within about 1-3 mm

Δ: Width of detached plating is within about 3-5 mm

X: Width of detached plating is about 5 mm or more

In Table 2, a hole expansion ratio (HER) is obtained by making a holehaving a diameter of 10 mm in a test piece having a size of 120×120 mm,expanding the hole using a punch having a forming portion angle of 60degrees until a crack is generated, and calculating a ratio of anexpanded hole to the initial hole of 10 mm in diameter. Also, in Table2, bending workability has been appraised by performing a bending teston a test piece using a 90 degree V-shaped punch, and measuring asmallest punch radius (mm) that does not cause breakage.

TABLE 1 Math Math FIG. 1 FIG. 2 Steel No. C Si Mn P S Al N Ti Nb Mo Cr BSb value value 1 0.06 0.1 2.5 0.01 0.004 0.035 0.005 0.02 0.05 0.03 0.90.0018 0.02 15.5 0.22 2 0.07 0.15 2.2 0.015 0.003 0.05 0.004 0.025 0.0550.05 0.7 0.0023 0.03 13.8 0.23 3 0.05 0.05 2.1 0.007 0.003 0.22 0.0030.015 0.045 0.01 0.5 0.0013 0.02 10.9 0.18 4 0.03 0.1 2.5 0.008 0.0040.043 0.005 — 0.06 — 0.7 0.0019 0.04 8.1 0.19 5 0.15 0.1 2.7 0.005 0.0030.052 0.003 0.04 — — 1.0 0.0021 0.03 11.7 0.31 6 0.08 0.5 2.1 0.0090.003 0.35 0.007 0.02 0.03 — 0.8 0.0032 0.04 18.0 0.23 7 0.07 0.20 2.10.012 0.003 0.04 0.003 — — 0.04 0.9 0.0017 0.02 17.5 0.22 8 0.15 0.2 2.70.015 0.008 0.043 0.005 — — — — 0.0012 — — 0.35 9 0.11 0.3 2.4 0.0150.008 0.043 0.005 — — 0.04 — — — — 0.30 10 0.18 0.2 1.8 0.011 0.0050.038 0.004 — 0.05 — — — — — 0.32 Math FIG. 1 = (Si/Mn + 150B)/SbMathFIG. 2 = C + Mn/20 + Si/30 + 2P + 4S When a value calculated by MathFIG. 1 is betwwen 5-20, and a value calculated by Math FIG. 2 is lessthan 0.27, an alloy design condition of the present invention issatisfied.

TABLE 2 Continuous Hole annealing Cooling Math Tensile expansion BendingSteel temperature rate FIG. 3 Plated Plating strength ratio workabilityBainite No. Product (° C.) (° C./sec) value appearance adhesion (MPa)(%) (mm) ratio Remark 1 CR 840 8 54.7 — — 1045 39 0 R 55 Steel of the 2GA 830 20 50.6 ◯ ⊚ 995 45 0 R 60 present invention 3 GA 855 20 30.8 ◯ ⊚830 68 0 R 45 4 GI 860 30 44.4 ◯ ⊚ 874 54 0 R 65 5 GA 810 20 62.8 ◯ ⊚1076 48 0 R 60 6 CR 820 10 62.1 — — 1012 60 0 R 55 7 GA 820 20 49.9 — —982 54 0 R 45 8 GA 810 20 — Non-plated X 1087 12 2 R 15 Steel ofcomparison 9 GA 810 20 — ◯ Δ 990 8 2 R 10 10 GA 810 20 — Non-plated Δ1040 7 2 R 10 Math FIG. 3 = −5LogCR + 25C − 17Si + 40Cr + 13,000B*Avalue calculated by Math FIG. 3 is 30 or more, the manufacturingcondition of the present invention is satisfied.

As shown in Table 2, when a steel sheet is manufactured according to themethod of the present invention, a high strength thin steel sheet havingtensile strength of about 800 MPa or more, having excellent surfacecharacteristic and mechanical characteristic compared to an existingcomparison material, and having excellent plating characteristic,welding characteristic, bending workability, and hole expansion ratiocan be manufactured.

According to the steel of the present invention, the steel sheet has astructure in which one selected from bainite and bainitic ferriteoccupies 40% or more, and ferrite and martensite occupy 25% or less and35% or less, respectively.

1. A high strength thin steel sheet having an excellent weldingcharacteristic, the thin steel sheet comprising: in weight %, C:0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% orless, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B:0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one materialselected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%,and Mo: 0.003-0.08%, and including Fe and other inevitable impurities asa remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27.
 2. The thin steelsheet of claim 1, wherein the steel sheet has a structure comprising atleast one selected from the group consisting of bainite and bainiticferrite occupying about 40% or more, and ferrite and martensite phasesoccupying the remainder.
 3. The thin steel sheet of claim 1, wherein thesteel sheet comprises a hot-dip galvanized (GI) layer or a galvannealed(GA) layer on a surface of the steel sheet.
 4. A method formanufacturing a high strength thin steel sheet having an excellentwelding characteristic, the method comprising: reheating a slab of thesteel sheet comprising, in weight %, C: 0.02-0.20%, Si: 1.5% or less,Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N:0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, andincluding at least one material selected from the group consisting ofTi: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Feand other inevitable impurities as a remainder, wherein Si, Mn, B, Sb,P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 andC+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at atemperature of a finish rolling exit side between the Ar₃ transformationpoint and 950° C.; pickling a wound hot rolled steel sheet andperforming cold rolling on the same at a reduction ratio of 40-80%; andperforming continuous annealing on an obtained cold rolled steel sheetat a temperature range of 740-860° C., cooling the cold rolled steelsheet down to 250-600° C. at a cooling rate satisfying a condition of −5Log CR+25C−17Si+40Cr+13,000B>30 in a cooling rate range of 3-150° C./s,and cooling the same at a cooling rate of 5° C./minute or more.
 5. Themethod of claim 4, wherein the steel sheet has a structure comprising atleast one selected from the group consisting of bainite and bainiticferrite occupying 40% or more, and ferrite and martensite phasesoccupying the remainder.
 6. The method of claim 4, further comprisingperforming hot-dip galvanizing (GI) or galvannealing (GA).